An image forming process performed in an electrophotographic image forming apparatus generally includes the steps of charging, exposing, developing, transferring, detaching, cleaning, charge removing, and fixing. In the step of charging, the surface of a photosensitive drum driven to rotate is charged evenly by a charging device. In the step of exposing, laser light is emitted onto the surface of the charged photosensitive drum so as to form an electrostatic latent image thereon. In the step of developing, a toner is electrostatically attached to the electrostatic latent image on the photosensitive drum so as to develop a toner image, thereby forming a toner image on the surface of the photosensitive drum. In the step of transferring, the toner image on the surface of the photosensitive drum is transferred by a transferring device onto a sheet. In the step of fixing, the toner image transferred onto the sheet is fixed to the sheet by a heating and fixing device. In the step of detaching, the sheet is detached from a heating roller of the heating and fixing device. In the step of cleaning, a toner remaining on the surface of the photosensitive drum is removed by a cleaning device and is recovered to a predetermined recovering section. In the step of charge removing, electric charges remaining on the surface of the photosensitive drum after the remaining toner has been removed is removed so as to prepare for a next image forming process.
In general, the photosensitive drum is a drum around which an organic photoconductor is applied as an optical conductive layer. A corona charger is widely used as the charging device for applying electric charges on the surface of the photosensitive drum. Examples of the corona charger include a wire corona charger and a sawtooth corona charger.
The wire corona charger includes a very thin conductive tungsten wire (discharging electrode) and a conductive shield case for covering portions other than an end of the tungsten wire (a portion facing the photosensitive drum). Application of a high voltage to the tungsten wire causes discharge from the tungsten wire to the photosensitive drum, so that the photosensitive drum is charged.
The sawtooth corona charger is provided with a sawtooth electrode (discharging electrode) including a plurality of acuminate protrusions and having a plate-like shape. The sawtooth corona charger discharges from the sawtooth electrode to the photosensitive drum, so that the photosensitive drum is charged.
The wire corona charger and the sawtooth corona charger can be used not only for charging the photosensitive drum but also for a charge removing device, a transfer device etc.
In the corona charger, when foreign matters attach to the discharging electrode, a portion to which the foreign matters attach drops its discharging performance. Consequently, the photosensitive drum cannot be evenly charged and charge unevenness occurs. The charge unevenness causes image defections such as black streaks on an obtained image. In order to solve the problem, Patent Citation 1 proposes a technique in which a cleaning roller for cleaning a discharging electrode is provided so as to remove foreign matters attached to the discharging electrode.
(Patent Citation 1)
Japanese Unexamined Patent Publication No. Tokukaihei 7-43990 (published on Feb. 14, 1995)
The technique proposed in Patent Citation 1 is effective for removing the foreign matters attached to the discharging electrode. However, the technique does not prevent a phenomenon that foreign matters attach to the discharging electrode. Therefore, in order to always assure a high-quality image with use of the technique of Patent Citation 1, it is necessary to frequently drive the cleaning roller. While driving the cleaning roller, printing cannot be performed. Consequently, performance of a printing device drops.
In consideration of the above, it is considered as follows: in order to prevent the image defects, it is more desirable to prevent attachment of foreign matters to the discharging electrode than to clean foreign matters attached to the discharging electrode as in Patent Citation 1.